Within the food packaging industry, packages formed from blanks of packaging material have been used for quite a long time. The packaging material typically comprises different layers of paper or board, liquid barriers of, for example, polymers and gas barriers of, for example, thin films of aluminium. The blanks are preformed from a material web, which sometimes is provided with a pattern of crease lines facilitating forming and folding of packages. The web is cut into pieces, with each piece having a size and shape for making one package. After cutting, each piece is folded into a flat tube-formed blank having its longitudinal edges overlapping each other. Next, the longitudinal edges are sealed by any appropriate, conventional sealing technology such as for example heat sealing. The result is a flat tube-formed blank. Forming a blank from a web is well known per se and will not be described in further detail.
In the packaging machine, the blank is raised to form a tube usually having a square or rectangular cross section depending on the type of package. Thereafter, one end of the tube can be transversally sealed forming a bottom (or top) of the package and the package is ready to be filled with a product, for example food products like, for instance, beverages. These kinds of packages are marketed by the applicant under the trade name Tetra Rex®.
There are also carton bottle packages made from a tube-formed sleeve of a packaging material, as described above, and a plastic top sealed to the sleeve. The tops are either pre-made outside the packaging machine or injection-moulded directly on the sleeve in the packaging machine. The tops are provided with closures. These types of packages can be filled before the bottom end of the sleeve is transversally sealed and final folded to a bottom. They can also be filled through the pour opening of the closure. Packages like this are marketed by the applicant under the trade names Tetra Aptiva® and Tetra Top®.
Partly formed packages that are open in one end for filling but sealed to form a bottom or top at the other end, are commonly denoted Ready-To-Fill packages (RTF packages). Hereinafter, ready-to-fill packages will be denoted with the wording “packages”.
To extend the shelf-life of the products being packed, it has been known to sterilize packages before the filling operation. Depending on the length of shelf-life desired and whether the distribution and storage is made at chilled or ambient temperature, different levels of sterilization can be chosen. However, the term sterilize as used herein comprises any level of cleaning and microbiological killing.
One way of sterilizing is to irradiate the inside of the package by electrons emitted from an electron beam unit. Such a method, and a device for realizing the method, is disclosed in International Application Publication No. WO 2005/002973, the disclosure of which is incorporated herein by reference.
An example of a system for sterilizing packages by electron beam technology includes an electron beam sterilizing device for emitting an electron beam along a path. The device is connected to an electron beam generator that is connected to a high voltage power supply and a filament power supply. The latter transforms power from the high voltage power supply to a suitable input voltage for a filament of the generator. The filament can be housed in a vacuum chamber. In operation, electrons e− from the filament are emitted along an electron beam path in a direction towards a target. A grid around the filament is used for diffusing the electron beam into a more uniform beam, and for focusing the electron beam towards the target. Beam absorbers and magnetic fields can also be used to shape the electron beam. The electrons exit the sterilizing device through an electron exit window.
However, when sterilizing packages, it has been found that it is difficult to achieve a uniform electron beam dose throughout the entire package with one electron beam sterilizing device. This is because of the different shapes of the physical portions of the package. Corners, openings, closures, bottle-like top portions, bottoms, flat walls and the like need to be sufficiently sterilized, but preferably without being over-exposed to irradiation. There is also a cost consideration involved in that it is preferable to not be forced to use more energy than necessary.